This invention relates to a process for producing single-crystal ceramics from polycrystal ceramics.
For the production of single-crystal ceramics, there are known the classical techniques such as Bridgman's method in which the molten material is cooled gradually from an end, and the Czochralski method in which the seed crystals are drawn up and used as nuclei for forming single crystals.
According to these classical fusion methods, high-temperature heating is required, and in the case of a material with an extremely high melting point such as ceramics, many difficulties are involved in the production process and it is also very difficult to obtain single crystals which are generally uniform in composition, and residual stress free so as to be worked easily.
These problems can be avoided by the partial melting method, but this method has the serious disadvantage that it is unable to produce forms of products other than very thin single crystal films.
Recently, there has been proposed a new partial melting method in which only part of a polycrystal body is melted to turn it into single crystal (Japanese Patent Publication No. 9278/86) and a solid-phase growth method in which high purity polycrystal ceramics are turned into single-crystal ceramics without melting the whole of the starting material (Japanese Patent Publication Nos. 1391/86 and 3313/86).
The solid-phase growth method has greatly improved the defects of the melting methods described above, but it requires a material of very high purity and also involves great difficulties in controlling the abnormal grain growth for making good use thereof. Stable solid-phase growth takes place as far as the grain growth occurs only at the interface 3 of single crystal 1 and polycrystal 2 as shown in FIG. 4(a), but among a host of polycrystal grains, there are many of those which become nuclei and make an abnormal growth as shown by 4, obstructing the normal grain growth as shown in FIG. 4(b). Ceramics are a product obtained by crushing, mixing and firing a mixture of materials, so that microscopic non-uniformity is a natural occurrence to them and perfect uniformity can hardly be expected of them. It has been disclosed that, in order to attain very extensive solid-phase growth, the workpiece must be maintained at a temperature which is only 10.degree. C. below the solid-phase growth temperature for a long time, but it is very difficult to maintain such uniform temperature of about 10.degree. C. below the solid-phase growth temperature. Further, ceramics tend to be contaminated by impurities in the step of mixing and exhibit increases in impurity concentration locally, and it is a well known fact that the higher the purity of the materials used, the stronger becomes the influence of such contaminants, resulting in more pronounced non-uniformity of the ceramics.